How to stop mtd char device generating ECC on writes?

[Jon Povey]

I am trying to write a binary image to NAND flash using nandwrite on TI DaVinci DM355. For historical reasons involving badly-written ROM bootloaders, I want to write the "legacy" OOB format for one area of flash (the UBL, second-stage bootloader).

For reference the legacy layout of a 2KB flash page is
  4x (512B data followed by 16B OOB (6 spare then 10 ECC)).

The "new" format used by u-boot, MTD etc. is
  2KB data followed by 64B OOB (24 spare then 40 ECC).

I can get a bootloader image with the needed (legacy) OOB and ECC using
  nanddump -f ubl0.dump -s 0x20000 -l 0x8000 -n /dev/mtd0

But I can't seem to write it back properly using nandwrite.
Testing on another partition mtd6, If I do this:
  nandwrite -r -s 0x20000 /dev/mtd6 ubl0.dump
then it seems my 64 bytes of "OOB" are skipped and I get a generated 24 bytes of 0xFF spare and 40 bytes of generated ECC.

If I use --noecc --oob instead of -r, it looks like my 64 bytes of "OOB" does get written, but then hardware ECC is getting written OVER THE TOP, i.e. the last 40 bytes have various bits 0 that should be 1 (but no 1s that should be 0)

I am looking through the nandwrite and drivers/mtd sources at the moment, but would love to get a clue if someone can tell me

- Does MTD support what I'm trying to do from userland?

- If yes, could this be a bug in the DaVinci NAND driver?

- Or, is nandwrite supposed to support this? or do I need to modify it?

Thanks folks,

--
Jon Povey
jon.povey at racelogic.co.uk

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